1. Field of the Invention
The present invention relates to a sheet sensing device which is used being loaded on, for example, a circulation type automatic document feeding apparatus (hereinafter referred to as "RDH") and which is used to sense that a sheet has made its one circulation.
2. Description of the Related Art
The RDH is used being loaded on, for example, a main body of a copying apparatus and the copying apparatus which is constructed including the RDH and the main body of the copying apparatus reads the image of a sheet-like document that is conveyed by the RDH and copies this image onto a prescribed sheet of recording paper. A reading portion for reading an image is set on the main body of the copying apparatus. Light is radiated onto the document that has reached the reading portion. The reflected light is imaged on a photosensitive body uniformly charged to a prescribed potential, forms an electrostatic latent image that corresponds to the image, develops this electrostatic latent image into a toner image, transfers this toner image onto a transfer sheet of paper, heats and fixes this transfer sheet of paper, and then discharges the resulting transfer sheet of paper.
In the RDH that includes a one-circulation sensing unit, the documents stacked are successively conveyed from above one by one and carried into a prescribed conveying path. The document has its image read at the reading portion of the main body of the copying apparatus and is returned to the bottom face of the stacked documents. This conveying operation is repeated whereby when the conveying operation is performed the one-circulation sensing unit senses whether or not the conveyance of the document has made its one circulation.
FIG. 16 is a side view illustrating the construction of a prior art one circulation sensing unit and FIG. 17 is a view illustrating a movable range 84 of a sensing member 85 of the one circulation sensing unit. To the bar-shaped sensing member 85 there is applied a power that is sent from a stepping motor 88 through a belt 89, whereby the sensing member 85 makes its rotary movement about a base end portion 85b thereof through the interior of a circular range whose radius is the length of the sensing member 85. A retracted position is set above the documents X stacked on a document tray 91 and a sensor 87 is disposed at this retracted position. Below the stacked documents X there is set a home position which is different from the retracted position and at which a sensor 90 is disposed. Also, the sensing member 85 is always urged toward the top face of the stack of documents X.
First, the sensing member 85 is disposed at its retracted position (S31). By driving the stepping motor 88 and thereby swinging the sensing member 85, this sensing member 85 is disposed at its home position (S32). That the sensing member 85 is disposed at its home position is sensed by the sensor 90. Further, the stepping motor 88 is driven and the sensing member 85 is thereby swung and is thereby caused to abut on the bottom face X2 of the stack of documents (S33), whereby the conveying operation of the documents is started. When the sensing member 85 is further swung whereby the sensing member 85 passes through the top face X1 of the stacked documents, the sensing member 85 is again disposed at its retracted position by the spring 86 (S31). That the sensing member 85 is disposed at its retracted position is sensed by the sensor 87.
The sensing member 85 makes its rotary movement about the base end portion 85b within the range of a circle whose radius is the length of the sensing member 85. Accordingly, the movement range of the sensing member 85 is relatively wide, with the result that the one-circulation sensing unit and hence the RDH becomes further enlarged in size.
Also, since the retracted position is set above the stacked documents X and the sensor 87 is disposed there and the home position that is different from the retracted position therebelow is set and the sensor 90 is disposed there, the number of the sensors is relatively large and the construction becomes complex.
Furthermore, since use is made of the stepping motor 88, the control thereof is relatively complex. Also, the manufacturing cost is relatively expensive.
Also, referring to FIG. 17, when the angle .theta.2 that is defined between a top face X1a, X1b of the stack of documents X that is located over the sensing member 85 and the locality thereon of the sensing member 85 is relatively large, in the mid-course of the conveying operation the sensing member 85 does not reliably abut on a face of the documents X, with the result that the sensing member 85 kicks up the documents X despite the documents X being still existent on the sensing member 85, with the result that the one circulation of the document conveyance is erroneously detected. In a case where conveying a large number of documents or conveying the documents each having a relatively large thickness, there increases the likelihood of such erroneous detection being made.
Although in order to prevent the occurrence of the above-mentioned inconvenience by making the angle .theta.2 small it is considered to make large the length of the sensing member 85, there arises the inconvenience of the one-circulation sensing unit being further enlarged in size.
A prior art disclosed in Japanese Unexamined Patent Publication JP-A 61-90930 (1986) relates to a document circulation apparatus which detects a failure of setting a document using a divider arm for sensing one circulation of document to control conveying documents. Specifically, the divider arm of the document circulation apparatus can make one turn about a prescribed axis, and after the divider arm is set on an uppermost document with a predetermined lever pin, the divider arm is fallen by its dead weight on conveyance of the documents.
A prior art disclosed in Japanese Unexamined Patent Publication JP-A 2-265874 (1990) relates to a circulation type document conveyance apparatus using a divider arm member for sensing a document circulation, specifically, in which the divider arm can turn horizontally and helically, and ascending and descending movements are conducted by a predetermined driving force and free fall, respectively.
Further, the prior art disclosed in Japanese Unexamined Patent Publication JP-A 5-265269 (1993) relates to a copying apparatus comprising sensing means for sensing one document circulation. A sensing actuator which is disposed at a position of the lowermost one of set documents ascends as the documents are conveyed, and when being positioned at a position of the uppermost document, the sensing actuator senses one document circulation. Such copying machine is disclosed in Japanese Unexamined Patent Publication JP-A 1-209233 (1989), too.
However, moving ranges of the divider arm disclosed in JP-A 61-90930, the divider arm member disclosed in JP-A 2-265874, and the sensing actuator disclosed in JP-A 5-265269 and JP-A 1-209233 are relatively wide the same as in an embodiment shown in FIG. 16, resulting in upsizing of the apparatuses.